Substrate bias pumps comprise an oscillating signal capacitively coupled at a pump node to a clamping means to level shift the signal downward and substrate coupling means for coupling the negative portion of the level shift signal to the substrate. Using conventional PN junction diodes for clamping the signal and coupling to the substrate is a problem for high density dynamic RAMs because of the energetic electrons which are thereby injected into the substrate and cause errors in the capacitive storage elements. In order to prevent such errors, diode-connected insulated gate field effect transistors (IGFETs) are used to bypass PN junctions because IGFETs can have a lower threshold voltage, for example 0.4 volt, than the PN junction voltage of 0.6 to 0.7 volt. This difference in voltage between the threshold voltage of an IGFET and a PN junction voltage drop also produces an increase in efficiency of the pump. The voltage drop of the clamp is reduced as is the voltage drop of the substrate coupling means. There may be cases, however, where transistors with a threshold voltage below 0.6 volt are not conveniently available. In any event, efficiency has been further improved by further reducing the voltage drop of the clamping circuit by a circuit shown in FIG. 1. There remains, however, the inefficiency of the voltage drop of the substrate coupling means as well as the possibility of electron injection for threshold voltages over 0.6 volt. In addition, there is also an inefficiency with a typical coupling capacitor. The typical capacitor is an IGFET with a gate as one electrode and a source and drain connected together as a second electrode at the pump node. The signal applied to the first electrode is reduced at the pump node by virtue of the parasitic capacitance of the source and drain to the substrate.